The invention relates to an electrically tunable, interferometric semiconductor laser including at least three segments which abut on one another in one plane, are monolithically integrated on a semiconductor wafer, are each composed of at least one laser-active zone and have a mirror at their free ends, the semiconductor laser further including a beam splitter by means of which the segments are connected with one another.
An interferometric semiconductor laser is a semiconductor laser configured according to the principle of a Michelson or Mach-Zehnder interferometer. Compared to a laser including a Fabry-Perot resonator, the interferometric semiconductor laser has the advantage that it emits in monomode operation while providing good suppression of side modes of the laser light. Moreover, when current is injected to change its optical length, this laser can be tuned through much more strongly than a laser including a DFB [distributed feedback] resonator. This makes it suitable for numerous applications: as a transmitting laser in multi-channel communications systems operating with wavelength multiplexing; as a local oscillator in coherent systems for the precise definition of channel wavelengths; and as a logic element. In the latter, the logic states "0" and "1" are represented by two wavelengths .lambda..sub.1, .lambda..sub.2.
From the publication entitled Appl. Phys. Lett. 52 (1988), pages 767-769, it is known to construct such a monolithically integrated, continuously tunable laser which is composed of at least three individual laser cavities that are controlled independently of one another by way of electrical contacts. Four segments acting as optical resonators form a cross-shaped laser at whose crosspoint a beam splitter is disposed at an angle of 45.degree. with respect to one of the segments.
A semiconductor laser including only three segments having different lengths L.sub.1, L.sub.2 2, L.sub.3 is constructed according to the same principle.
Such a semiconductor laser is produced on a (100) GaAs/GaAlAs double heterostructure by metal-organic gas phase epitaxy in which the segments are grown photolithographically in the [011] and [110] directions. Then the segments are coated with a dielectric material and electrodes are attached, the segments are metallized on the rear of the semiconductor substrate and split at their ends.
By bombardment with a gallium ion beam, narrow troughs are created at the point of connection of the three segments so as to form the beam splitter. The troughs extend down to a distance of 0.1 .mu.m from the laser-active zone so that a non-negligible coupling is created between the segments.
Compared to the mode spectrum of an individual laser, the interferometric semiconductor laser, due to the different lengths L.sub.1, L.sub.2, L.sub.3 of the three segments, results in greater amplification of every n.sup.th mode as follows: ##EQU1##
In spite of the relatively long lengths L.sub.1, L.sub.2, L.sub.3 of the individual segments, only the difference between the lengths of the laser cavities (L.sub.2 +L.sub.3)-(L.sub.1 +L.sub.2) plays a role in the separation of the modes, while modes of a frequency difference of c/2L occur in a laser including a Fabry-Prot resonator, where c is the speed of light and L the length of the laser cavity. Only if the laser is sufficiently short, will it oscillate at a single frequency. However, this interferometric semiconductor laser has the drawback that the frequency of the light it emits cannot be changed.